PCI: iproc: Add DT docs for Broadcom iProc PCIe driver

Document the Broadcom iProc PCIe platform interface device tree binding.

Signed-off-by: Ray Jui <rjui@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>